A load circuit for supplying electric power to a load such as a lamp or a motor mounted on a vehicle includes a battery and a semiconductor relay (MOSFET or the like) provided between the battery and the load. The battery, the semiconductor relay, and the load are connected through conductors including wires. The load circuit further includes a control circuit for on/off operations of the semiconductor relay. According to drive/stop signals outputted from the control circuit, the semiconductor relay is turned on/off to switch between drive and stop of the load.
In such a load circuit, if excess current flows through the load circuit, it is necessary to instantly break the load circuit to protect the load, wire, semiconductor relay, and the like. Accordingly, a method is proposed which measures load current at each predetermined sampling period; estimates wire temperature based on a period of time when the load current flows; and when the estimated temperature reaches a previously set threshold temperature, turns off the semiconductor relay (see PTL 1).
FIG. 1 is a characteristic diagram illustrating changes in wire temperature estimated by using a temperature estimation method disclosed in PTL 1. In FIG. 1, if the semiconductor relay is turned on at time t21, the wire temperature gradually increases (see a curve q11). After the wire temperature reaches a saturation temperature for normal current, the wire temperature is stabilized at this saturation temperature. At this time, the estimation of the increasing wire temperature is performed at intervals of a predetermined sampling period dt.
If the semiconductor relay is turned off at time t22, the wire starts to release heat at this point of time, and the temperature thereof gradually decreases to reach the ambient temperature. Even after the semiconductor relay is turned off, it is therefore necessary to calculate heat released from the wire and estimate the temperature of the wire. Accordingly, the calculation logic for the wire temperature executes the calculation until the wire temperature reaches the ambient temperature. If the logic to calculate the wire temperature is not executed while the wire is releasing heat, it is determined that the wire temperature is equal to the ambient temperature when the semiconductor relay is again turned on before the wire temperature decreases to the ambient temperature. In this case, the subsequent estimation of temperature cannot be executed. At this time, estimation of the temperature of wire releasing heat is executed at intervals of the predetermined sampling period dt.